1. Field of the Invention
The invention relates to an electrode-connecting lead used for semiconductor devices and a method for fabricating the lead.
2. Description of the Background Art
Silicon semiconductors are widely and popularly used as semiconductors. Aluminum pad electrodes are usually used for terminals connecting these semiconductors and external electrodes, because they have good processibility and superior characteristics including low electric resistance, and low-cost producibility.
Various structures were devised to connect the aluminum pad electrode on a silicon semiconductor element to an external terminal. Conventionally, the wire-bonding method is mainly used, in which the aluminum pad electrode is connected to an independent metallic lead terminal, called a lead frame, with a wire of gold or aluminum. As semiconductor elements become highly integrated, the number of electrode increases and the electrode itself is subdivided, causing dense lead terminals. In response to these circumstances, instead of an independent electrode such as a lead frame, a lead of metallic film formed on a polyimide film by etching or other methods is used as a lead to connect directly to the pad electrode on a silicon semiconductor. An example of this means is disclosed in Unexamined Japanese Patent Publication 62-199022, in which a copper bump electrode provided on a metallic lead through a hole in the polyimide film on the lead is connected to the pad electrode formed on a silicon semiconductor element, the bump electrode being plated with gold to prevent the copper part from rusting. In this example, a desirable result is obtained when the pad electrode on the silicon element is made thinner than the insulating passivation film on the element.
Unexamined Japanese Patent Publication 3-206633 discloses a method in which a bump electrode need not be attached to a lead, because the aluminum pad electrode on a silicon element protrudes beyond an insulation film. According to the application, it is desirable that copper or aluminum be used as the lead; it is more desirable that the lead surface be plated with aluminum or the lead surface be plated with nickel before plating with aluminum.
Unexamined Japanese Patent Publication 4-323837 discloses a method in which a copper lead is connected to the pad electrode on a silicon element through an aluminum electrode of a tape carrier package (TCP), avoiding a direct connection of the copper lead to the pad electrode. In other words, the connection between the lead and the element is made possible by using aluminum or aluminum alloy for the inner lead. In this example, an aluminum-plated copper-based lead is adhered to a polyimide film, for instance, and the end portion is processed into the inner lead by etching or other methods.
In order to fulfill high reliability in the connection between the aluminum pad electrode on a silicon element and the lead, various methods were devised as shown above. However, further improvement is needed to reduce the manufacturing cost at the same time.
The fundamental method of the conventional devices is based on the connection between the aluminum pad electrode on a silicon element and the lead, mainly consisting of copper, formed on a polyimide film. When copper is used for the lead, it is difficult to connect the copper lead to the aluminum pad. The reason is that whereas the oxide film on the surface of the aluminum can be broken by the application of ultrasonic waves so that the activated aluminum underneath the oxide film directly contacts the copper for satisfactory connection it is difficult to break the oxide film on the surface of the copper by ultrasonic waves. To circumvent this difficulty, a bump electrode is provided on the copper lead. In a foregoing prior art, the copper bump electrode is plated with gold. In another example cited above, instead of using the bump electrode, the copper lead is plated with aluminum. Thus, one solution is to use aluminum for the lead to be connected with the aluminum pad electrode on a silicon element. However, to secure a more reliable connection, it is necessary to thoroughly examine the aluminum used in the lead and the interface between the aluminum bump electrode and copper lead.